1. Field of the Invention
The invention relates to an interferometer system for measuring the position and displacement of an object in a plane which is parallel to the XY plane of an XYZ system of co-ordinates, by X and Y measuring mirrors which are arranged on a holder for the object. The system is being provided with or a generator which generates a plurality of measuring beams and for directing or directs the measuring beams along a plurality of measuring axes which are parallel to the XY plane and extend to and from the measuring mirrors, and radiation-sensitive detectors for converting measuring beams reflected by the measuring mirrors into electric measuring signals. The number of X and Y measuring axes are at least equal to the number of object movements to be measured interferometrically.
The invention also relates to a lithographic projection apparatus which includes such an interferometer system. This apparatus may be a stepper or a step-and-scanner.
A measuring axis of the interferometer system is an axis along which the position of the displacement in a given direction (X or Y) of a given point of the object is measured. This measuring axis does not need to coincide with the chief ray of the measuring beam which is used for this measurement. If the measuring beam is sent through the system twice and reflected twice at substantially the same point by the object, the measuring axis is situated between the chief ray of the measuring beam on the first passage and the chief ray of this beam on the second passage.
2. Description of the Related Art
EP-A 0 498 499 describes an embodiment of an interferometer system, and an optical lithographic projection apparatus including such an interferometer system, for repetitively reduced imaging of a mask pattern, for example, the pattern of an integrated circuit (IC) on a substrate provided with a radiation-sensitive layer. Between two successive images of the mask pattern on the same substrate, the substrate and the mask are moved with respect to each other, for example, parallel to the X or Y direction of an XYZ system of co-ordinates, while the substrate plane and the mask plane are parallel to the XY plane.
The projection apparatus is used in combination with masking and diffusion techniques for manufacturing integrated circuits. In this method, a first mask pattern is imaged in a large number, for example, one hundred IC areas of the substrate. The substrate is then removed from the projection apparatus to subject it to the desired physical and/or chemical process steps. Subsequently, the substrate is arranged in the same or another; similar, projection apparatus to form an image of a second mask pattern in the different IC areas of the substrate, and sc forth. It should then be ensured that the images of the mask patterns are positioned very accurately with respect to the substrate fields.
To this end, the projection apparatus which are currently used comprise:
an interferometer system for the substrate table with which the movements of this table, and hence those of the substrate holder and the substrate, can be followed accurately and the position of the substrate can be determined accurately; PA1 an alignment system with which the mask can be aligned with respect to the substrate, which alignment system closely co-operates with the interferometer system; PA1 a focus error detection system with which it can be ensured that the images of the mask pattern formed in IC areas of the substrate are always sharp, and PA1 a tilt detection system for detecting the tilt of the substrate and the image of the mask pattern with respect to each other, which detection system may be combined with the focus error detection system and is then also referred to as focus-and-level detection system.
The known composite interferometer system may have three measuring axes with which the movements of the substrate in the X direction and the Y direction and the rotation .phi..sub.z of the substrate about the Z axis, being the optical axis of the projection system, can be measured. The special feature of the interferometer system in accordance with EP-A 0 498 499 is that it has five measuring axes and that not only the displacement of the substrate along the X axis and the Y axis and the rotation about the Z axis can be measured very accurately, but also the tilt .phi..sub.x about the X axis and the tilt .phi..sub.y about the Y axis. When this interferometer system is used, each field of the substrate can be positioned very accurately with respect to the mask pattern without a separate alignment per field being necessary. Consequently, the time required for illuminating a substrate can be reduced considerably.
In the optical lithographic projection apparatus, hereinafter also referred to as photolithographic apparatus, the image field of the projection system must coincide with the substrate surface, i.e., there should be a given relation between the Z positions of the image plane and those of the surface of the substrate holder. In the currently used photolithographic apparatus, in which the substrate table is always positioned underneath the projection lens system and is moved across distances which, at a maximum, on the order of the substrate size, this relation is controlled by means of said focus-and-level detection system whose elements are arranged in a measuring frame in the form of a plate which is rigidly connected to the projection system. With the aid of the above-mentioned detection systems and the servosystems of which they form part, the substrate and the mask pattern can then not only be positioned globally with respect to each other, but also sufficiently accurately per field. In a novel generation of photolithographic projection apparatus currently being developed, with the aid of which ICs having a larger number of components must be manufactured, i.e., with which even smaller details must be imaged in the substrate fields and in which the substrate table is moved across distances which are larger than the substrate size, new problems present themselves. On one hand, there is the problem of the necessity of an even more accurate alignment, and, on the other hand, there is the problem that the relation can no longer be controlled in the manner described. Consequently, there is a need for another mode of measuring the distance, in the Z direction, between the projection system and the substrate holder which is also referred to as chuck.